1. Field of the Invention
The present invention relates to wireless communication, and more particularly, a band pass filter and method of filtering for a base station in a wireless communication system.
2. Description of Related Art
The current evolution in wireless networking is being driven by the increasing demand for high bandwidth mobile applications, such as the mobile internet. The resulting trends in development of hardware and software are posing a need for significant reduction in cost, size, and power consumption of base station transceivers (BTS). Consequently, innovations in BTS designs are expected to contribute to a substantial reduction of size and cost.
One such method to achieve these reductions is to change the architecture of the RF transmitter. Instead of the conventional heterodyne architecture (multistage up conversion), a direct conversion architecture offers a simplified and smaller design at a lower cost by eliminating IF (intermediary frequency) based components.
Because of the simplified architecture, a common design can be used across different frequency platforms, such as UMTS, PCS and Cellular. This would further reduce the cost in producing a radio transmitter.
However, because of process technologies, there is a limitation on the achievable noise output at the direct conversion modulator. Any noise produced by the modulator would directly end up at the transmitter output. If the transmitter was always sending out the same carrier frequency, a band pass filter could be used to reduce the noise floor outside the pass band.
A typical BTS, however, has to be able to transmit signals over different frequencies. For example, the TX (transmission) band in UMTS applications is 2110 to 2170 MHz. Furthermore, there is a stringent requirement for UMTS on the noise output 50 MHz away from the center of a transmitted signal. A signal being transmitted at 2110 MHz would need a much different filter than one that is being transmitted at 2170 MHz to meet the spurious noise requirements of UMTS.